The need for speed-bandwidth has affected literally all aspects of electronic design. Many designers have responded by taking advantage of the inherent speed of NPN transistors, as compared to PNP transistors. NPN transistors circuit have improved speed and overall gain. However, despite the inherent advantages of NPN transistors, high gain RF amplifiers using them continue to couple amplifier stages with capacitors (AC coupling), decreasing amplifier efficiency by placing an additional load in the output circuits of each stage.
Beyond that, DC coupling in integrated circuits requires the fabrication and connection of capacitors in close proximity to other components on the same substrate, creating so-called parasitic capacitance, which degrades amplified performance and stability in an unpredictable way, complicating circuit layout.